Various medical devices, such as catheters, tubes, balloons, stents and the like, are known to have physical performance requirements which change at particular points, or ranges of area or length. For instance, catheters typically need to be soft and flexible toward the distal end while at the same time becoming much more rigid and kink resistant proximally in order to effectively transmit torque and crossing forces from their proximal ends to the distal tip.
Medical devices comprising catheter balloons are used in an increasingly widening variety of applications including vascular dilatation, stent delivery, drug delivery, delivery and operation of sensors and surgical devices such as blades, and the like. The desired physical property profile for the balloons used in these devices vary according to the specific application, but for many applications a high strength robust balloon is necessary and good softness and trackability properties are highly desirable.
Commercial catheter balloons have been formed of a wide variety of polymeric materials, including PET, nylons, polyurethanes, polyolefins, and various block copolymer thermoplastic elastomers.
U.S. Pat. No. 4,490,421, Levy, and U.S. Pat. No. 5,264,260, Saab, describe PET balloons. U.S. Pat. No. 4,906,244, Pinchuk et al, and U.S. Pat. No. 5,328,468, Kaneko, describe polyamide balloons. U.S. Pat. No. 4,950,239, Gahara, and U.S. Pat. No. 5,500,180, Anderson et al describe balloons made from polyurethane block copolymers. U.S. Pat. No. 5,556,383, Wang et al and U.S. Pat. No. 6,146,356, Wang et al, describes balloons made from polyether-block-amide copolymers and polyester-block-ether copolymers. U.S. Pat. No. 6,270,522, Simhambhatla, et al, describes balloons made from polyester-block-ether copolymers of high flexural modulus. U.S. Pat. No. 5,344,400, Kaneko, describes balloons made from polyarylene sulfide. All of these balloons are produced from extruded tubing of the polymeric material by a blow-forming radial expansion process. U.S. Pat. No. 5,250,069, Nobuyoshi et al, U.S. Pat. No. 5,797,877, Hamilton et al, and U.S. Pat. No. 5,270,086, Hamlin, mention still further materials which may be used to make such balloons.
It has been found that polymers with a high content of butylene terephthalate can crystallize so extensively from an extrusion melt that balloon formation from an extruded parison is very difficult, if possible. A solution to this problem, taught in U.S. Pat. No. 6,465,067, Wang et al, is to add boric acid to the polymer composition.
In commonly owned copending U.S. application Ser. No. 10/055,747, medical devices formed of thermoplastic polymers containing chain extension additives which increase polymer molecular weight are described.
In commonly owned copending U.S. application Ser. No. 10/087,653, filed Feb. 28, 2002, incorporated herein by reference, it is disclosed that improved balloon properties can be obtained by controlling the parison extrusion in a manner which restricts the elongation of the parison material in the longitudinal direction. The application discloses that decreasing the gap between the extrusion head and the cooling bath tank can lower parison elongation by shortening the quench time.
In commonly owned copending U.S. application Ser. No. 10/617,428, filed Jul. 10, 2003, it is taught that varying the cooling tank gap during an extrusion can provide a catheter tube or balloon parison which has variable properties along its length.
In a balloon catheter, heat welded balloon-to-tube bonds, typically provided by laser heating, are commonly used for their high reliability. However, heat welded bonds provide a new problem, the melted or softened regions of the joined parts will often resolidify relatively slowly, allowing crystallization to develop with consequent increased stiffness. At the distal end of the catheter where the balloon is typically bonded to the catheter inner tube, the increased crystallinity in the bond can adversely affect the desired softness and trackability and of the catheter tip. Selecting a slow crystallizing polymer for the balloon material is usually not a suitable option since balloon material selection and processing steps are typically directed to maximizing balloon wall strength and hence providing a high degree of crystallization.
At the same time the catheter distal outer tube near the site, where it is bonded to the proximal waist of the balloon, often is subjected to very high tensile stress when the balloon is collapsed after use and is being withdrawn into a guide catheter or a protective sleeve. In some cases, particularly with larger balloons, the catheter shaft immediately proximal of the balloon may begin to yield before the balloon is successfully withdrawn. Consequently the tensile strength of the catheter outer can limit the minimum guide catheter or sleeve diameter which may be used with the catheter.